I have a few ways, they are pretty specific to Toronto but I'll mention them anyway so you get the idea.
1) Redflagdeals.com - this is my primary source as it is the largest forum in Canada (with a huge center around Toronto). Basically it is a "deals" forum like slickdeals or fatwallet in the states, but there is a TON of talk about things to do in Toronto, great restaurants, and of course, deals on stuff. For me I browse this often and if I find something I like then I'll check it out. For example, I found out about "O'Noir" here, which was a restaurant where you eat in pitch black, and all the waiters are blind.
2) Local blogs - things like blogto.com show me a lot of the independent type stuff around (festivals, smaller shops, bands, etc.). Torontoist is another one. For example I just learned they are opening a Ping Pong bar in downtown Toronto from here.
3) Deal sites (like groupon, living social, etc). Basically it isn't just about buying the deal (as I only buy maybe 1 per week) but I keep an eye out for the new things to do that they feature - I might find a paintball place and keep it in mind even if I don't buy the deal, or some sort of trade show/exhibition, etc.
For things like a) sports and b) music I'd basically either know that it was coming (I'm a huge sports fan with seasons tickets to baseball and basically never miss a leafs game) so I don't really have to search out that info.
I try to be pretty frugal with what I do so that is certainly a bias - If I'm going to a new city the first thing I do is save the Wikitravel page to my iPad, then I'll check out some of the past groupons there, etc. I'll then check out some of the online coupon places (for example I got coupons for 2-4-1 mini-golf & bowling in Florida, etc). Last resort when I get there I'll check out the rack cards at tourist type places (in Florida I found out about a pirate tour boat, lol).
If I were you I'd be skimming forums as your best bet for crowd sourcing data, there are two different opinions for "things to do" one being a local and one being a tourist, but usually locals don't do enough tourist stuff, and tourists don't do enough local stuff... so there is an interesting middle you could find there (for example, I've never been to the Royal Ontario Museum, or the Bata Shoe Museum after living in Toronto for 5 years - these would be done by 75% of tourists). Of course you could skim foursquare data too but it seems to be done already.
Good luck
Thanks for the feedback
Edit: checked out your project, looks very similar to http://usehipster.com, which was recently acquired (?) and it hasn't even launched. So, theres confirmation of the idea.
Horseback riding, trap shooting, skiing, fishing, and that kind of thing. A site that gives me something to do on the weekends when I'm in the mood for an activity.
We also have simple and clean filters... already have 200+ submissions just from my friends.
We're also planning to put in a bunch of great plug ins.
Chris